Electrical automobiles require reduction in size and an improvement in reliability of power semiconductor devices and an inverter unit using the same. For the reduction in size and improvement in reliability of the power semiconductor devices and inverter unit, it is necessary to increase cooling efficiency of the power semiconductor devices and inverter unit.
Hereinafter, a conventional general inverter unit is described with reference to FIGS. 7 to 9, and a main portion of a publicly-known inverter unit with cooling efficiency increased compared to the general inverter unit is described with reference to FIGS. 10 and 11.
FIG. 7 is a plan cross-sectional view of the conventional general inverter unit; FIG. 8 is a side cross-sectional view thereof; and FIG. 9 is a cross-sectional view of a main portion showing the inverter unit with the power semiconductor devices attached thereto.
In FIGS. 7 and 8, the inverter unit includes a power semiconductor device 2, an aluminum electrolytic capacitor 4 as a power supply smoothing capacitor, current detectors 101 and 102, and a control unit 11. The power semiconductor device 2 is attached to a bottom face of an inverter unit case 1 by attachment screws 3. The aluminum electrolytic capacitor 4 is fixed to a fixed base 5. The current detectors 101 and 102 detect currents of three-phase output conductors 91 to 93.
The power semiconductor device 2 and aluminum electrolytic capacitor 4 are electrically connected to a positive conductor 7 and a negative conductor 8 by connection screws 6. In the bottom face of the inverter unit case 1, a flow passage 13 is provided, and the power semiconductor device 2 is cooled by a refrigerant 12 flowing within the flow passage 13. The refrigerant 12 is, for example, antifreeze.
As shown in FIG. 9, the power semiconductor device 2 has a layered structure which includes a radiating metal plate 14 attached to upper part of the flow passage 13; an insulating substrate 15 joined to upper part of the radiating metal plate 14; a metal electrode 16 joined to upper part of the insulating substrate 15; and an IGBT 171 and a diode 181 joined to upper part of the metal electrode 16. The IGBT 171, diode 181, metal electrode 16, and insulating substrate 15 are normally accommodated in an insulating resin package. The resin package is bonded to the radiating metal plate 14. Within the resin package, insulating gel is encapsulated.
Furthermore, on the rear surface of the radiating metal plate 14 of the power semiconductor device 2, heat transfer grease 19 is applied. The heat transfer grease 19 reduces contact thermal resistance generated when the power semiconductor device 2 is attached to the bottom face of the inverter unit case 1, for which the flow passage 13 is provided, by the attachment screws 3.
In operation of the thus structured power semiconductor device 2, heat loss is generated when the IGBT 171 and diode 181 are energized. Inside the resin package accommodating the IGBT 171 and diode 181, the insulating gel as a heat insulating material is encapsulated as described above. Accordingly, most of heat generated from the IGBT 171 and diode 181 is conducted to the metal electrode 16, which is provided under the IGBT 171 and diode 181. The heat conducted to the metal electrode 16 is conducted to the radiating metal plate 14 through the insulating substrate 15. As shown in FIGS. 7 to 9, the radiating metal plate 14 is, as previously described, pressed into contact with the bottom face of the inverter unit case 1 by the attachment screws 3 with the heat transfer grease 19 interposed therebetween. The heat generated in the IGBT 171 and diode 181 is therefore released by the refrigerant 12.
The above described conventional inverter unit includes the following problems.
First, in the conventional inverter unit, the radiating metal plate 14 is pressed into contact with the bottom face of the inverter unit case 1, which includes the flow passage 3, using the attachment screws 3 in the periphery of the power semiconductor device 2. Accordingly, the pressing force is applied only around the attachment screws 3 and not uniformly applied to the entire radiating metal plate 14. Although the heat conducing grease 19 is applied to the rear surface of the radiating metal plate 14 in order to reduce the contact thermal resistance, heat cannot be uniformly transferred from the entire rear surface of the radiating metal plate 14 to the flow passage 13. In addition, thickness of the insulating substrate 15, which is provided under the IGBT 171, is thin. Accordingly, the heat generated in the IGBT 171 cannot be sufficiently diffused within the insulating substrate 15. The contact thermal resistance between the radiating metal plate 14 and the inverter unit case 1 is therefore greatly increased to the level equivalent to thermal resistance within the power semiconductor device 2, thus providing a lower cooling efficiency.
Second, since the thickness of the insulating substrate 15, which is provided under the IGBT 171, is thin, thermal time constant (heat capacity) thereof is small. Accordingly, at inverter activation which is controversial because of a large increase in temperature of the IGBT 171 and diode 181, transient thermal resistance is large, and such a large increase in temperature cannot be suppressed.
The Japanese Patent Laid-open Publication No. 2003-153554 has proposed an inverter unit with the cooling efficiency increased by solving the above described problems of the general inverter unit. This inverter unit disclosed in the Japanese Patent Laid-open Publication No. 2003-153554 is described using FIGS. 10 and 11.
FIG. 10 is a partial longitudinal cross-sectional view of the inverter unit described in the Japanese Patent Laid-open Publication No. 2003-153554, especially showing a mounting structure of a semiconductor chip within a power semiconductor device. FIG. 11 is a partial perspective view of the power semiconductor device of the inverter unit.
In the inverter unit shown in FIG. 10, each arm of a three-phase inverter is composed of a plurality of IGBTs 171 and diodes 181, which are semiconductor chips, connected in parallel to each other. Each of these semiconductor chips is configured to have a square planar shape not more than 10 mm on a side. These semiconductor chips are joined to a conductor 20, whose thickness is not less than 1.5 mm and not more than 5 mm, and the conductor 20 is bonded to a cooler 22 by an insulating resin sheet 23 containing ceramics.
As shown in FIG. 11, in the inverter unit described in the Japanese Patent Laid-open Publication No. 2003-153554, each arm of the three-phase inverter is configured to have a mounting structure of semiconductor chips shown in FIG. 10. Four IGBTs 171A to 171D connected in parallel and two diodes 181A and 181B connected in parallel, which constitute a W-phase upper arm, are arranged in a line on an upper arm conductor 25, which constitutes the upper arms of the three-phase inverter. In a similar manner, four IGBTs 172A to 172D connected in parallel and two diodes 182A and 182B connected in parallel, which constitute a W-phase lower arm, are arranged in a line on a lower arm conductor 26, which constitutes the lower arms of the three-phase inverter. Furthermore, between the upper and lower arm conductors 25 and 26, a three-phase output conductor 27 is disposed. The three-phase output conductor 27 connects the IGBTs 171A to 171D and diodes 181A and 181B, which are arranged on the upper arm conductor 25, to a three-phase output terminal 32. In the example shown in FIG. 11, the lower arm conductor 26 and three-phase output conductor 27 are formed of a same conductor. Furthermore, between the upper and lower arm conductors 25 and 26, a negative conductor 28 is disposed. The negative conductor 28 connects the IGBT 172A to 172D and diodes 182A and 182B, which are arranged on the lower arm conductor 26, to a negative terminal 31. The IGBTs and diodes are electrically connected to each conductor by bonding wires 29.
In the inverter unit described in the Japanese Patent Laid-open Publication No. 2003-153554 and shown in FIGS. 10 and 11, entire surfaces of the IGBTs 171A to 171D and diodes 181A to 181C, which are joined to the conductor 20 and upper and lower arm conductors 25 and 26, are directly bonded to the cooler 22 by use of the insulating resin sheet 23. Accordingly, there is no contact thermal resistance in a portion in contact with the cooler unlike the conventional general power semiconductor device shown in FIG. 9, and the thermal resistance of the IGBT and diode chips within the power semiconductor device is reduced by half. Furthermore, the IGBT 171A to 171D and diodes 181A to 181C are joined to the conductor 20 and upper and lower arm conductors 25 and 26, whose thicknesses are not less than 1.5 mm and not more than 5 mm. Accordingly, the thermal time constant is increased because of an effect of the thermal capacities of the conductor 20 and upper and lower arm conductors 25 and 26 to reduce the transient thermal resistance, and the increase in temperature at the inverter activation becomes small. The cooling efficiency is therefore increased, and the inverter unit can be reduced in size.